1. Field of the Invention
This invention relates to a crane comprising a hoisting mechanism, which comprises a hoisting rope and a hoisting rope winch.
2. Description of the Prior Art
When a crane is used to hoist a load, special problems will arise when the load to be lifted from a support performs vertical movements relative to the crane. For instance, when a crane which is fixedly mounted on a drilling platform is used to hoist loads from a ship, said loads will perform vertical movements in unison with the ship relative to the crane on the platform. Said vertical movements will be due to the rolling and pitching performed by the ship in dependence on the amplitudes and periods of the waves. Depending on the wind and other weather conditions, said movements may be of considerable magnitude. When loads are being hoisted from a ship performing such movements, the latter will exert considerable dynamic stress shocks on the hoisting rope and via the hoisting rope on the crane structure. Said stress shocks are taken into account by the shock allowance factor Cb, which can be calculated by the following equation: EQU Cb=1+0.9.times.(vh+vw).times.(K/(g.times.L)).sup.1/2
wherein
Cb=shock allowance factor, i.e., the factor by which the magnitude of the nominal load is to be multiplied for the calculation and design of the crane PA0 vh=velocity of crane hook PA0 vw=design vertical velocity of the load-carrying deck of a supply ship PA0 K=spring constant of the crane related to the vertical displacement of the load hook PA0 g=gravitational constant PA0 L=force action of the payload
It is apparent that the shock allowance factor depends on the hoisting velocity, the stiffness of the crane and the vertical velocity of the load to be hoisted relative to the crane and takes the dynamic stresses into account which are exerted on the crane by the shock action of the load moving relative to the crane. In dependence on the significant height of the waves and the mean period of the waves that shock allowance factor may lie between 1.3 and about 4.5. Because a crane for hoisting moving loads must be designed for the largest shock allowance factor which may be required, the expenditure involved in the crane structure is greatly increased by the shock allowance factor.